The present invention relates to a process for the preparation of a solvent zone for use in the production of ternary or quaternary semiconductor compounds by the so-called "solvent zone transfer method".
This process is suitable for producing ternary compounds of formula Cd.sub.x Hg.sub.0.5-x Te.sub.0.5 and quaternary compounds of formula Cd.sub.x Hg.sub.0.5-x Te.sub.y Se.sub.0.5-y, whereby in said formulas 0&lt;x&lt;0.5 and 0&lt;y&lt;0.5. These compounds have a forbidden band width varying continuously with the atomic fractions x and y, which makes it possible to produce infrared photo detectors operating at any wavelength exceeding 0.8 .mu.m.
From the general standpoint, the solvent zone transfer method makes it possible to produce monocrystalline semiconductor compounds at a temperature below their melting point, which makes it possible to obtain very pure compounds, which have a better crystalline quality than those obtained using other monocrystal production processes. In the particular case of semiconductor compounds of formula Cd.sub.x Hg.sub.0.5-x Te.sub.0.5 or formula Cd.sub.x Hg.sub.0.5-x Te.sub.y Se.sub.0.5-y, said method makes it possible to overcome the difficulties associated with the presence of mercury vapor having a high pressure at the melting point of said compounds.
The basic principle of the so-called solvent zone transfer method applied to semiconductor compounds of type II-VI has been clearly described in the article by G. A. WOLFF et al, which appeared in the Transactions of the Metallurgical Society of AIME, vol. 242, March 1968, pp 436-441.
With reference to FIG. 1, said method consists of introducing a polycrystalline semiconductor compound ingot, called the source ingot 2 into a cylindrical silica boat 4 in the verticle position, where it is traversed by a molten solvent zone 6, positioned beforehand at the end of the boat. A furnace 8 makes it possible to heat the boat and its content in the portion facing the furnace. The longitudinal displacement of the boat, in accordance with arrow 10, enables the solvent zone 6 to dissolve the ingot 2 and to crystallize behind it a monocrystalline ingot 12. The dissolving interface carries reference 13 and the recrystallization interface reference 14.
The dissolving of the source ingot 2 by the solvent zone 6 leads to a modification of the initial composition of the solvent zone. The growth process of ingot 12 is established when the solvent zone 6, which is liquid, is in thermodynamic equilibrium with the source ingot 2 and the monocrystallized ingot 12.
In the aforementioned article, the method is applied to the growth of monocrystals of ternary semiconductor compounds of type II-VI. The initial solvent zone, ie the solvent zone existing before any transfer of said zone along the semiconductor ingot is made from pure tellurium.
In the same way, in the article by R. TRIBOULET, which appeared in the Revue de Physique Appliquee, Vol. 12, pp 123-128, February 1977 a description is given of the preparation of crystals containing Cd, Hg, Te from juxtaposed CdTe and HgTe bars constituting the source or supply materials of an initial tellurium solvent zone.
The choice of an initial solvent zone made from pure tellurium imposes the temperature of the final solvent zone from which it is possible to obtain the crystallization in the form of a monocrystal of the supply compound of said zone, in view of the fact that the liquid solvent zone and the solid compound must be in thermodynamic equilibrium.
In order to make it possible to choose the temperature at which the monocrystal could form, it has been proposed to use as the initial solvent zone, a solvent zone having the final composition of said zone, ie when the latter is in thermodynamic equilibrium with the supply compound and the crystallized compound.
French Pat. No. 8 105 387 of 18.3.1981 describes such a solvent zone. This patent, which refers to obtaining semiconductor compounds containing Cd, Hg and Te, uses as the initial solvent zone a mixture of CdTe, HgTe and Te, which is rich in tellurium, said solvent zone being supplied by CdTe and HgTe bars. The preparation of a tellurium-rich, ternary, initial liquid zone containing Cd, Hg and Te in thermodynamic equilibrium with the corresponding solid compound, involves preparing the solvent zone at the highest possible thermodynamic equilibrium temperature and guarding against mercury leaks linked with the high vapour pressure of said constituent.
Moreover, in the case where the cadmium is introduced in the form of a CdTe compound, the preparation of the solvent zone takes several hours, bearing in mind the slow dissolving kinetics of said compound in the HgTe+Te liquid.
Thus, in the various prior art processes, significant problems occur in connection with the preparation in the form of monocrystals of ternary or quaternary compounds, particularly of formulas Cd.sub.x Hg.sub.0.5-x Te.sub.y and Cd.sub.x Hg.sub.0.5-x Te.sub.y Se.sub.0.5-y, when using the solvent zone transfer method.
Thus, to obtain monocrystals of pure semiconductor compounds with a high crystalline quality, it is necessary to produce the initial solvent zone at as moderate a temperature as possible, said zone at the best being able to reach the thermodynamic equilibrium with the solid taking the place of the supply compound of said zone.